The basic precast prestressed pavement concept consists of a series of individual precast panels that are post-tensioned together in the longitudinal direction after installation on site. Each of the panels are pretensioned in the transverse direction (long axis of the panel) during fabrication, and ducts for longitudinal post-tensioning are cast into each of the panels. The basic panel layout consists of three types of panels...
Base panels make up the majority of the post-tensioned pavement section and are placed between the joint panels and central stressing panels. All of the panels have continuous tongue and groove keyways along the edges of the panels. The pretensioning strands extend along the length (long axis) of the panels, and the post-tensioning ducts are oriented across the width (short axis) of the panels.
The joint panels are located at the ends of each post-tensioned section of pavement. The joint panels contain dowelled expansion joints which "absorb" the expansion and contraction movements of the post-tensioned section. Each half of the joint panel is tied to the panels on either side of the actual expansion joint. The joint panels also contain the post-tensioning anchorage for the longitudinal post-tensioning tendons. The anchors are cast into the joint panels on either side of the expansion joint. Blockouts or pockets cast into the joint panels provide access to the post-tensioning anchors.
Central Stressing Panels
Post-tensioning is completed at the central stressing panels. The post-tensioning strands are fed into the ducts at the large blockouts cast into the central stressing panels. Strands are fed in either direction from each blockout down to the anchors in the joint panels. The strands from either side of the blockout are then coupled together and tensioned.
Variations of the post-tensioning procedure have already been utilized on demonstration projects completed to date. One such variation utilized for a demonstration project in Missouri eliminated the central stressing panels and all post-tensioning was completed from the anchor access blockouts in the joint panels.
Providing a flat, stable platform for the precast panels to rest on is important. While the panels do have the ability to span over voids and "soft" base materials, providing full support beneath the panels is ideal. Thus far, both hot-mix asphalt and lean concrete have been utilized for the underlying base material. The precast panels tend to settle into "softer" materials such as hot-mix asphalt, reducing potential voids beneath the pavement.
Regardless of the base material used, some form of friction-reducing material is needed between the precast pavement and base material. Because the precast pavement system consists of long sections of post-tensioned pavement, significant expansion and contraction movement will occur on a daily basis with changes in temperature. The friction-reducing material prevents the precast pavement and base from bonding to one another, and reduces frictional restraint to slab movement.
As described above, tongue and groove keyways are cast into the edges of the precast panels. These keyways help ensure vertical alignment between panels as the panels are installed. The keyways also provide temporary load transfer between panels prior to post-tensioning. It should be noted that the panels are not match-cast. The tolerances of the keyway dimensions are such that the panels can be fabricated on a long-line casting bed without the need to match-cast. What's more, match-casting requires the panels to be installed in a very specific sequence, whereas "standard" panels can be utilized for long-line fabrication.
A highly viscous epoxy is normally applied to the keyways prior to assembling the panels. The epoxy acts as a lubricant during panel installation and also seals the joints between panels to prevent the intrusion of water and incompressibles. The epoxy also bonds the panels together so that they act as a continuous slab after post-tensioning.
As described above, the joint panels contain the expansion joints at the end of each post-tensioned section of pavement. Up to four inches of movement can be expected at the expansion joints, so they must be robust and able to withstand repeated heavy wheel loading. Both armored joints (similar to those used for bridge decks) and plain dowelled joints have been utilized on projects to date. The type of joint will depend on the expected traffic loading and movement of the pavement slab. Regardless of the type of joint used, dowels across the joint are essential for providing load transfer.